Category Archives: Fortnightly Weather Review

CR58

April 23, 2026

Subseasonal Weather Outlook (27 April – 10 May 2026)

Issued: 22 April 2026
First forecast week: 27 April – 3 May 2026
Second forecast week: 4 – 10 May 2026

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are predicted over much of the southern Maritime Continent in Week 1 (27 April – 3 May).

Drier than usual conditions are predicted over much of central and southern Mainland Southeast Asia and parts of the western Maritime Continent in the next fortnight (27 April – 10 May).

Warmer than usual temperatures are predicted over much of central and southern Mainland Southeast Asia and most of the northern half of the Maritime Continent in the next fortnight (27 April – 10 May).

A Madden-Julian Oscillation (MJO) signal was present over the Western Hemisphere and Africa (Phases 8 and 1) during mid-April. Most models predict the MJO signal to continue propagating eastwards, reaching the Indian Ocean (Phases 2 and 3) by the start of the forecast period, and then weakening over the Indian Ocean during Week 1 (27 April – 3 May).

The outlook is assessed for the region in general, where conditions are relative to the average conditions for the corresponding time of year. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.

April 17, 2026

Review of Regional Weather for March 2026

1. Overview

1.1 During March 2026, below-average rainfall was recorded over much of the Maritime Continent with a mix of below and near-average rainfall over most of Mainland Southeast Asia (Figure 1). Mix of below- to above-average (wetter) anomalies were also observed over parts of the eastern equatorial region for both datasets (GSMaP-NRT and CMORPH-CRT). There were dry anomalies over much of central and southern Mainland Southeast Asia, with some regions of above-average rainfall over parts of Viet Nam and northern Myanmar. The largest positive (wetter) anomalies were recorded over Papua for GSMaP-NRT, and over northern Myanmar and western Sumatra for CMORPH-CRT, with the largest negative (drier) anomalies over northwestern Borneo for both datasets.

1.2 The observed rainfall anomaly pattern of below-average rainfall over the Maritime Continent, where the highest anomalies are recorded, is consistent with the predictions from the subseasonal weather outlooks for March 2026 (2 – 15 March 2026 and 16 – 29 January 2025). The subseasonal weather outlooks predicted a drier central Maritime Continent, which is more consistent with the recorded rainfall from CMORPH-BLD. The below-average rainfall over the Maritime Continent is also consistent with the seasonal rainfall outlook for March, with higher confidence of below-normal rainfall over the central region predicted.

Figure 1: Rainfall anomalies for March 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over parts of northeastern Mainland Southeast Asia, with below- to near-average temperature for southeastern and northwestern Mainland Southeast Asia (Figure 2). Below-average temperature was recorded for the northeastern and southeastern Maritime Continent, while near- to above-average temperature was recorded for western and central Maritime Continent. The warmest anomalies (2°C – 3°C above average) were recorded over northern Viet Nam, while the coolest anomalies (0.5°C – 1°C below average) were recorded over eastern Myanmar, northern Philippines and eastern Cambodia.

Figure 2: Temperature anomalies for March 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was inactive for much of March based on the RMM diagram (Figure 3). The signal was inactive during the beginning of March, then emerged shortly in Week 3 over the Western Pacific (Phase 7), propagating to Western Hemisphere (Phase 8) and then decayed over the Western Hemisphere (Phase 8) in the same week. The MJO signal remained indiscernible for much of Week 4, before emerging over the Western Hemisphere (Phase 1) at the end of the month. At this time of the year, Phases 7 and 8 tend to bring drier conditions to the Maritime Continent, with the observed rainfall in March also showing drier conditions.

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

2.2 The El Niño – Southern Oscillation (ENSO) condition is “Neutral”. The Indian Ocean Dipole (IOD) is currently in the neutral state.

March 13, 2026

Review of Regional Weather for February 2026

1. Overview

1.1 During February 2026, above-average rainfall was recorded over much of the northern half of the Maritime Continent with near-average rainfall over most of Mainland Southeast Asia (Figure 1). While above-average (wetter) anomalies are observed distributed over much of northern half of the area for both datasets (GSMaP-NRT and CMORPH-BLD), there is disagreement elsewhere over the Maritime Continent. GSMaP-NRT recorded above-average anomalies but CMORPH-BLD recorded mainly below-average (drier) anomalies for much of the southern half of the Maritime Continent. There were no notable anomalies over most of Mainland Southeast Asia, in line with the current dry season, with some regions of above-average rainfall over parts of Viet Nam. The largest positive (wetter) anomalies were recorded over southern Philippines for both datasets, with the largest negative (drier) anomalies over southern Borneo for CMORPH-BLD.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the northeastern Maritime Continent, where the highest anomalies are recorded, is consistent with the predictions from the subseasonal weather outlooks for February 2025 (2 – 15 February 2026, and 16 Febraury – 1 March 2025). The subseasonal weather outlooks predicted a drier central Maritime Continent, which is more consistent with the recorded rainfall from CMORPH-BLD. The observations match the most likely tercile from the seasonal outlook for February 2026 for northeastern Maritime Continent where the rainfall observed the largest positive anomalies.

Figure 1: Rainfall anomalies for February 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over most of Mainland Southeast Asia, with below- to near-average temperature for southeastern Mainland Southeast Asia (Figure 2). For the Maritime Continent, near-average temperature was generally recorded. The warmest anomalies (2°C – 3°C above average) were recorded over northern Viet Nam, while the coolest anomalies (0.5°C – 1°C below average) were recorded over Papua and central Cambodia.

Figure 2: Temperature anomalies for February 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of January, the Madden-Julian Oscillation (MJO) was inactive based on the RMM diagram (Figure 3). The signal emerged over the Western Pacific (Phase 6) in Week 2, strengthening with little eastward propagation in Phase 6 until middle of Week 3 when it weakened slightly and started again propagating eastwards. By Week 4, the MJO signal was in Phase 7 of the Western Pacific where it stalled and weakened at the end of the month. At this time of the year, Phases 6 and 7 tend to bring drier conditions to the western and southern Maritime Continent, in line with the observed rainfall in January also showing drier conditions for much of the Maritime Continent.

Figure 3: The Madden-Julian Oscillation (MJO) was inactive for much of February based on the RMM diagram (Figure 3). The signal decayed over the Western Hemisphere (Phase 8) at the beginning of the month , before emerging shortly in the middle of February over the Indian Ocean (Phases 2 and 3) and decaying again after a week. The MJO signal remained indiscernible for much of Week 4, before emerging over the Maritime Continent (Phases 4 and 5) at the end of the month. At this time of the year, Phases 2 and 3 as well as Phases 4 and 5 tend to bring wetter conditions to the Maritime Continent, with the observed rainfall in February also showing wetter conditions (data from the Bureau of Meteorology, Australia).

2.2 La Niña conditions persisted and continued weakening in February 2026. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) is currently in the neutral state.

February 20, 2026

Review of Regional Weather for January 2026

1. Overview

1.1 During January 2026, below- to near-average rainfall was recorded over much of the Maritime Continent with near-average rainfall over Mainland Southeast Asia (Figure 1). For the Maritime Continent, most of the equatorial region recorded below-average (drier) anomalies, with scattered spots of near- to above-average (wetter) anomalies for the southern and northeastern regions for both datasets (GSMaP-NRT and CMORPH-CRT). There were no notable anomalies over Mainland Southeast Asia, in line with the current dry season. The largest positive (wetter) anomalies were recorded over central Philippines, with the largest negative (drier) anomalies over western Borneo.

1.2 The observed rainfall anomaly pattern of below-average rainfall over much of the equatorial region and the above-average rainfall over northeast Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for January 2025 (22 December 2025 – 4 January 2026, 5 – 18 January 2025, and 19 January – 1 February 2026). The observations match the most likely tercile from the seasonal outlook for January 2026 for central and northeastern Maritime Continent where the rainfall observed the largest drier and wetter anomalies, but differ for much of the rest of the Maritime Continent.

Figure 1: Rainfall anomalies for January 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over the eastern Maritime Continent and parts of northern Mainland Southeast Asia, while below-average temperature was recorded over central and southern Mainland Southeast Asia, with near-average temperature for much of the rest of Southeast Asia (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over western Myanmar, while the coolest anomalies (1°C – 2°C below average) were recorded over Cambodia, southern Viet Nam and central Thailand.

Figure 2: Temperature anomalies for January 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.2 La Niña conditions persisted in January 2026, although key atmospheric indicators show signs of the La Niña conditions weakening. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) is currently in the neutral state.

January 23, 2026

Review of Regional Weather for December 2025

1. Overview

1.1 During December 2025, a mix of below- to above-average rainfall was recorded over the Maritime Continent with near-average rainfall over Mainland Southeast Asia (Figure 1). For the Maritime Continent, most of the equatorial region recorded above-average (wetter) anomalies with the surrounding region recording predominately below-average (drier) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended). There were no notable anomalies over Mainland Southeast Asia, in line with the current dry season. The largest positive (wetter) anomalies were recorded over parts of Peninsular Malaysia and Borneo, with the largest negative (drier) anomalies over the northern Malay Peninsula and the southern Philippines.

1.2 The observed rainfall anomaly pattern of below-average rainfall over parts of the Maritime Continent outside of the equatorial region is consistent with the predictions from the subseasonal weather outlooks for December 2025 (24 November – 7 December 2025, 8 – 21 December 2025, and 22 December – 4 January 2026) although the above-average anomalies for most of the equatorial region were not predicted. The observations differ from the most likely tercile from the seasonal outlook for December 2025, which predicted an increase in chance of above-normal rainfall over parts of the northeastern and southern Maritime Continent.

Figure 1: Rainfall anomalies for December 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over northeastern Mainland Southeast Asia and the northeastern Maritime Continent, with near-average temperature for much of the rest of Southeast Asia (Figure 2). The warmest anomalies (1°C – 3°C above average) were recorded over northern and central Viet Nam, while the coolest anomalies (0.5°C – 1°C below average) were recorded over northern Myanmar.

Figure 2: Temperature anomalies for December 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of December, the Madden-Julian Oscillation (MJO) was present over the Western Pacific (Phase 7) based on the RMM diagram (Figure 3). The signal propagated eastwards, weakening over Western Hemisphere (Phase 8) and entering the unit circle by Week 2. For the rest of December there was no sustained MJO signal based on the RMM index. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

2.2 La Niña conditions persisted in December 2025. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) returned to neutral in December 2025.

December 22, 2025

Review of Regional Weather for November 2025

1. Overview

1.1 During November 2025, a mix of below- to above-average rainfall was recorded for Southeast Asia (Figure 1). For the Maritime Continent, most of the northeastern and northwestern regions recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), with a mix of below- to above-average rainfall recorded over the equatorial region. Over Mainland Southeast Asia, above-average rainfall was recorded mainly over the central and eastern regions. Generally, CMORH-Blended (Figure 1, right) recorded more extensive negative (drier) anomalies over the equatorial region. The largest positive (wetter) anomalies were recorded over the northwestern Maritime Continent , with the largest negative (drier) anomalies over central Borneo.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the Mainland Southeast Asia and below-average rainfall over the central Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for November 2025 (27 October – 9 November 2025, 10– 23 November 2025, and 24 November – 7 December 2025).The observations are also somewhat consistent with the seasonal outlook for November 2025, which predicted an increase in chance of above-normal rainfall over most of Mainland Southeast Asia and the northeastern Maritime continent, with an increase in chance of below-normal rainfall over the central Maritime Continent.

Figure 1: Rainfall anomalies for November 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average temperatures were recorded over most of Mainland Southeast Asia in November 2025, while near- to above-average temperatures were recorded over most of Maritime Continent (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over east central Sumatra, while the coolest anomalies (0.5°C – 1°C below average) were recorded over central Mainland Southeast Asia.

Figure 2: Temperature anomalies for November 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of November, the Madden-Julian Oscillation (MJO) was present over the Maritime Continent (Phase 5) based on the RMM diagram (Figure 3). It stalled over the Western Pacific (Phase 6) during Week 2 and then weakening in Phase 6 during the third week. The MJO strengthened again in Phase 6 towards the end of Week 3 and resumed its eastward propagation, reaching phase 7 in Week 4. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

2.2 A negative Indian Ocean Dipole (IOD) was present in November 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1. La Niña conditions were also present. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.

November 26, 2025

Review of Regional Weather for October 2025

1. Overview

1.1 During October 2025, a mix of below- to above-average rainfall was recorded for Southeast Asia (Figure 1). For the Maritime Continent, most of the southern and eastern regions recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), with below-average rainfall recorded over the western region and parts of the Philippines. Over Mainland Southeast Asia, below-average rainfall was recorded over the southern region, with near- to above-average rainfall elsewhere. Generally, CMORH-Blended (Figure 1, right) recorded more extensive negative (drier) anomalies over the western Maritime Continent and southern Mainland Southeast Asia. The largest positive (wetter) anomalies were recorded over eastern Mainland Southeast Asia, with the largest negative (drier) anomalies over the western Maritime Continent.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the southern and southeastern Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for October 2025 (1 – 12 Oct 2025 and 13 – 26 Oct 2025). However, the below-average rainfall observed was not predicted in the subseasonal weather outlook. The observations are also somewhat consistent with the seasonal outlook for October 2025, which predicted an increase in chance of above-normal rainfall in the southern and eastern Maritime Continent, and an increase in chance of below-normal rainfall in the western Maritime Continent.

Figure 1: Rainfall anomalies for October 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Near- to above-average temperatures were recorded over most of Southeast Asia in October 2025, with the exception of below-average temperatures recorded over Papua (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over northern Myanmar, while the coolest anomalies (0.5°C – 1°C below average) were recorded over northern Papua.

Figure 2: Temperature anomalies for October 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of October, the Madden-Julian Oscillation (MJO) decayed over the Indian Ocean (Phase 2) and remained inactive in Week 1 and start of Week 2 based on the RMM diagram (Figure 3). An MJO signal then emerged over the Western Hemisphere (Phase 1) towards the end of Week 2, propagating eastwards to the Indian Ocean (Phases 2 and 3) in Week 3 and to the Maritime Continent (Phases 4 and 5) in Week 4. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

2.2 A negative Indian Ocean Dipole (IOD) was present in October 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1. La Niña conditions were also present. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.

October 17, 2025

Review of Regional Weather for September 2025

1. Overview

1.1 During September 2025, many parts of Southeast Asia experienced above-average rainfall (Figure 1). Most of the Maritime Continent recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), although CMORH-Blended (Figure 1, right) recorded more extensive negative (drier) anomalies over the northwestern part of the region . Over Mainland Southeast Asia, positive anomalies were recorded over the eastern and central parts of the region. Over the western parts, the two datasets disagree, with GSMaP-NRT recorded mostly above-average rainfall while CMORPH-Blended recorded near-average rainfall over this region. The largest positive (wetter) anomalies over Southeast Asia were recorded over northeastern Mainland Southeast Asia and much of the Philippines.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for September 2025 (1 – 14 Sep 2025 and 15 – 28 Aug 2025). The subseasonal weather outlooks also predicted a mix of below-and above-average rainfall over parts of Mainland Southeast Asia, in line with the observations. The observations are also somewhat consistent with the seasonal outlook for September 2025, which predicted an increase in chance of above-normal rainfall in the southern and eastern Maritime Continent.

Figure 1: Rainfall anomalies for September 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Near- to above-average temperatures were recorded over Mainland Southeast Asia in September 2025, while below- to above-average temperatures were recorded over the Maritime Continent (Figure 2). The coldest anomalies (0.5°C – 1°C below average) were recorded over southern Sumatra and northern Philippines, in line with the above-average rainfall anomalies. The warmest anomalies (0.5°C – 1°C above average) were recorded over parts of northeast and northwest Mainland Southeast Asia, as well as over Sulawesi.

Figure 2: Temperature anomalies for September 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of September, the Madden-Julian Oscillation (MJO) was inactive based on the RMM diagram (Figure 3). An MJO signal emerged over the Indian Ocean (Phases 2 and 3) towards the end of the first week of the month, before rapidly decaying in the middle of the second week and then remaining inactive until the end of the month. At this time of the year, Phases 2 and 3 tend to bring drier wetter conditions to the western Maritime Continent. However, these patterns are not visible in Figure 1, indicating other drivers likely had a stronger influence on the region’s rainfall.

2.2 A negative Indian Ocean Dipole (IOD) was present in September 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1.

September 15, 2025

Review of Regional Weather for August 2025

1. Overview

1.1 During August 2025, many parts of Southeast Asia experienced above-average rainfall (Figure 1). Most of the Maritime Continent recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), although CMORH-Blended (Figure 1, right) recorded negative (drier) anomalies over much of the western Maritime Continent, and parts of Borneo, the southeastern Maritime Continent and northern Philippines. Over Mainland Southeast Asia, positive anomalies were recorded over the east , with a mix of below- to above-average rainfall elsewhere. The largest positive (wetter) anomalies over Southeast Asia were recorded over eastern Mainland Southeast Asia and the southeastern Maritime Continent.

1.2 The observed rainfall anomaly pattern of above-average rainfall over much of the Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for August 2025 (4 – 17 Aug 2025 and 18 – 31 Aug 2025). The subseasonal weather outlooks also predicted a mix of below-and above-average rainfall over parts of Mainland Southeast Asia, in line with the observations. The observations are also somewhat consistent with the seasonal outlook for August 2025, which predicted an increase in chance of above-normal rainfall in the southern and eastern Maritime Continent.

Figure 1: Rainfall anomalies for August 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Near- to above-average temperatures were recorded over Mainland Southeast Asia in August 2025, while below- to near-average temperatures were recorded over most of the Maritime Continent (Figure 2). The coldest anomalies (0.25°C – 1°C below average) were recorded over Borneo and the southeastern Maritime Continent, in line with the above-average rainfall anomalies. The warmest anomalies (0.5°C – 2°C above average) were recorded over parts of central Mainland Southeast Asia, Sumatra, and Java.

Figure 2: Temperature anomalies for August 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At start of August, a Madden-Julian Oscillation (MJO) signal was present in the Western Hemisphere (Phase 8) based on the RMM diagram (Figure 3). This signal propagated eastwards, reaching the Indian Ocean (Phases 2 and 3) by the second week of August. However, as the signal approached the Maritime Continent (Phase 4) in the last week of August, the signal rapidly weakened and became indiscernible. At this time of the year, Phases 8 and 1 tend to bring drier conditions to southern Mainland Southeast Asia and the Philippines, and to a lesser extent, Phases 2 and 3 can bring wetter conditions to the western Maritime Continent. However, these patterns are not visible in Figure 1, indicating other drivers likely had a stronger influence on the region’s rainfall.

2.2 There were signs of a negative Indian Ocean Dipole (IOD) developing in August 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1.

August 21, 2025

Review of Regional Weather for July 2025

1. Overview

1.1 During July 2025, Southeast Asia experienced a mixed of below- to above-average rainfall, with the northwestern and parts of the central Maritime Continent as well as southern Mainland Southeast Asia experiencing below- to near-average rainfall while western and northern Mainland Southeast Asia and northern Philippines experienced above-average rainfall (Figure 1). The largest positive (wetter) anomalies were recorded over northern Philippines. However, there were some disagreements between GSMaP-NRT and CPC Unified Gauge datasets, with CPC Unified Gauge recording more intense below-average rainfall over Borneo and southern Mainland Southeast Asia.

1.2 The observed rainfall anomaly pattern of below-average rainfall over the Maritime Continent and southern Mainland Southeast Asia is mostly consistent with the predictions from the subseasonal weather outlooks for July 2025 (7 – 20 July 2025 and 21 July – 3 August 2025). The subseasonal weather outlooks also predicted the above-average rainfall over parts of Mainland Southeast Asia. The observations are also somewhat consistent with the seasonal outlook for July 2025, although the extent of the predicted region for below-normal rainfall is much smaller than the observed rainfall.

Figure 1: Rainfall anomalies for July 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average and near-average temperatures were recorded over much of northern Mainland Southeast Asia in July 2025, while above-average temperatures were recorded over most of the Maritime Continent and southern Mainland Southeast Asia (Figure 2). The coldest anomalies (-1°C – -2°C below average) were recorded over central Myanmar while the warmest anomalies (2°C – 3°C above average) were recorded over central Sumatra.

Figure 2: Temperature anomalies for July 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 There was no discernible Madden-Julian Oscillation (MJO) signal at the start of July based on the RMM diagram (Figure 3). In the middle of Week 2, an MJO signal developed over the Maritime Continent (Phase 4) which then propagated eastwards to the Western Pacific (Phases 6 and 7) by the end of Week 3. The MJO signal continued to propagate through the Western Pacific before decaying at the end of the month. At this time of the year, Phases 4 and 5 tend to bring wetter conditions to parts of the Maritime Continent while Phase 7 tends to bring drier conditions.

2.2 ENSO-neutral conditions were present in July 2025.